1. Field of the Invention
This invention relates to an electric wind-up device of a camera.
2. Description of the Prior Art
Motor drive devices having a torque detecting mechanism which senses the terminus of a camera film or detects an increase in torque upon completely drawing the film from the film magazine to automatically de-energize the motor which is operating to draw the film are known, for example, from U.S. Pat. No. 4,112,449 or others.
Upon the increase in torque, a substantial force is instantaneously applied between the perforations of the film and the sprocket of the camera. Some kinds of film may be broken down at the perforations by such a substantial force under a circumstance of low temperature, particularly at temperature of -10.degree. C. or below. In an extreme case, the film may be severed completely such that it cannot be rewound in the magazine.
In order to overcome such problems, it is desirable to use a motor de-energizing means actuated by a counter which can be preset in accordance with the number of film frames to be used, in addition to another motor de-energizing means actuated upon detecting an increase in torque. In this case, the motor can be de-energized by means of the frame counter upon completion of winding up a predetermined number of frames to be used when the camera is used at low temperature or when it is desired to terminate the winding-up after the predetermined number of frames has been used.
On the other hand, there is also known a method in which the film is automatically rewound by utilizing the wind-up motor in the motor drive device. In such a method, one-way clutches are provided at the respective inputs of gear trains for wind-up and rewind so that the gear trains can be automatically switched from one to another by selecting the rotational directions in the motor. However, this method is not very appropriate in that it requires a complicated switch means for changing over the rotational directions in the motor and an expensive structure having large size as well as less durability.
Another method utilizes a rewind lever or the like which can be operated externally to switch a clutch mechanism in such a manner that gear trains for wind-up and rewind can be individually driven without reversing the motor. The last-mentioned method provides an inexpensive structure having smaller size and increased durability.
If an automatic rewind device including the last-mentioned clutch mechanism is combined with the motor drive device utilizing the two motor de-energizing means above-mentioned in such a manner that either of these de-energizing means can be selected to terminate the wind-up of film, there is an inconvenience in such an operation mode that the motor is de-energized upon counting a predetermined number of film frames. If the motor is de-energized to terminate the wind-up of film at the terminus thereof upon detecting an increase in torque, the clutch switching mechanism in the motor drive device will not in any case be affected by the reaction in the camera because of de-energizing the motor. However, if an operation mode in which the motor is de-energized upon counting a predetermined number of film frames is selected, the de-energization of the motor due to the torque detecting mechanism cannot be caused when the mechanical phase in de-energizing the motor does not coincide with the torque detecting phase, so that the wind-up operation will be terminated and will cause a frictional force between the power transmitting surfaces in the clutch mechanism under the reaction in the camera resulting in larger operating force or uncomfortable feeling in switching the clutch.